1. Field of the Invention
The present invention relates to a fixing unit of an electrophotographic apparatus.
2. Description of the Related Art
FIG. 3 is an outline configuration diagram of an electrophotographic apparatus having the compressed air supply device.
As illustrated in the drawing, the surface of a photosensitive element 1 that is rotated at a constant rotation speed in the direction indicated by the arrow is uniformly charged by a charging unit 7, and a laser beam is emitted from an exposure unit 8 onto the surface of the charged photosensitive element 1 to form an electrostatic latent image corresponding to the image information to be recorded. The electrostatic latent image is visualized with toner supplied by a developing unit 18 to form a toner image 5 on the surface of the photosensitive element 1.
When a sheet 6 fed at an appropriate timing passes through a gap between the photosensitive element 1 and a transfer unit 2 by being in contact with the photosensitive element 1, the toner image 5 on the photosensitive element 1 is transferred to the sheet 6. The sheet 6 to which the toner image 5 is transferred is electrostatically peeled off from the photosensitive element 1 by a peeling unit 3 and is conveyed to a fixing unit 10 by a conveying belt 4. In general, the sheet 6 is conveyed to the fixing unit 10 by being sucked by the conveying belt 4.
The toner image 5 on the sheet 6 fed to the fixing unit 10 is fixed to a surface of the sheet 6 by heat and pressure at a fixing nip 9 including a heating roller 11 and a pressing roller 12.
Due to a problem that the sheet 6 having passed through the fixing nip 9 tightly sticks, via the melted toner, to the heating roller 11 and is therefore not conveyed, Japanese Patent Application Laid-open No. 2009-300704 proposes a technique, for example, by which the sheet 6 is peeled off from the heating roller 11 with compressed air A that blows a leading end of the sheet 6.
In FIG. 3, an injection nozzle 13 spouts the compressed air A toward the sheet 6; the compressed air A is supplied to the injection nozzle 13 through a second pipe 14b; a compressed air injection solenoid valve 15 is provided in the middle of the second pipe 14b; an upper guide plate 20 and a lower guide plate 21 guide the sheet 6 peeled off from the heating roller 11 to the direction of a discharging unit (not illustrated).
FIG. 4 is an outline configuration diagram of a compressed air generator included in a fixing unit illustrated in Japanese Patent Application Laid-open No. 2009-300704.
The compressed air generated by a compressor 17 is allowed to pass through a first pipe 14a to be reserved in an air tank 16. The air tank 16 is connected with the injection nozzle 13 by the second pipe 14b through the compressed air injection solenoid valve 15. The solenoid valve 15 is a normally closed solenoid valve, and the pressure in the air tank 16 is maintained with the solenoid valve 15 being closed.
When a leading end of the sheet 6 reaches a predetermined position, the compressed air injection solenoid valve 15 is opened for a predetermined period of time, and the released compressed air A is spouted from the injection nozzle 13 to peel off the sheet 6 from the heating roller 11, thus preventing the sheet from sticking to the heating roller 11.
However, if the pressure of the compressed air is too weak, the sheet 6 cannot be reliably peeled off. Conversely, if the pressure of the compressed air is too strong, the sheet 6 is buckled or the posture of the sheet 6 is off-balanced, resulting in a trouble in conveying the sheet. Therefore, it is necessary to properly adjust and maintain the pressure, and accordingly, a pressure adjusting valve 19 is provided to the air tank 16. The pressure adjusting valve 19 allows a surplus of the compressed air generated by the compressor 17 to be released to the atmosphere, so that the whole pressure of a compressed air circuit can be appropriately maintained.
It should be noted that a method and an apparatus for automatically discharging a droplet are described in, for example, Japanese Patent No. 3581960.
When the air is compressed, the dew-point temperature of the air increases. When the temperature of the compressed air falls below the dew-point temperature, water vapor contained in the air is condensed to turn into a droplet. In general, it has been known that when the air is continuously compressed and the droplets are not drained from the compressed air circuit, the droplets are reserved in the compressed air circuit to cause a trouble.
If the droplets are reserved in the compressed air circuit in the conventional fixing unit illustrated in FIG. 4, the droplets are spouted from the injection nozzle 13 together with the compressed air, and the sheet 6 gets wet, thus causing the print quality to be markedly deteriorated. Furthermore, the droplet washes away grease used in the compressed air injection solenoid valve 15 to shorten the durable life of the solenoid valve 15.